1. Field of the Invention
The present invention relates to a canister that has activated carbon as adsorption material and a heater for heating the activated carbon. The canister is preferable for an evaporative emission control system for vehicle.
2. Description of Related Art
In a vehicular evaporative emission control system, a canister containing activated carbon is used for adsorbing fuel vapor. The canister is communicated with a fuel tank via a vapor line. The canister is arranged to be able to communicate with atmosphere for introducing purge air when the canister is desorbed. The canister is also communicated with an intake passage of an engine via a purge line. A purge valve is disposed on the purge line.
The activated carbon for the canister has average pore radius in a range of about 12.0 Angstroms to 14.0 Angstroms, and particle diameter in a range of about 1.6 mm (millimeter) to 3.0 mm. The canister further comprises means for heating the activated carbon for desorption. Such a heating technique is effective to enhance adsorption and desorption performances of the activated carbon. JP-U-5-21158, JP-A-60-6061, and JP-U-2-13161 disclose canisters that have heater means.
However, adsorption performance of the conventional canister is not enough to satisfy several requirements, because the conventional activated carbon has relatively large average pore radius. The adsorption performance can be lowered due to a residual heat, because the activated carbon has relatively large particle size and has less heat conductance.
It is an object of the present invention to provide a canister that has improved adsorption and desorption performances.
According to a first aspect of the present invention, the canister has a heating means that heats activated carbon particles when desorption. The activated carbon particles have pore volume of 0.28 ml/ml (milliliters/milliliter) or more. The activated carbon particles have average pore radius in a range of 10.5 Angstroms to 12.0 Angstroms. The pore volume and the average pore radius are measured by the nitrogen adsorption Cranston-Inkley method.
The activated carbon particles obtain high adsorption performance since the pore volume is 0.28 ml/ml or more and the average pore radius is relatively small in a range of 10.5 Angstroms to 12.0 Angstroms. The pore volume of 0.28 ml/ml or more is needed to provide high adsorption performance.
FIG. 7 is a graph showing n-butane working capacities at 25xc2x0 C. (Celsius degrees), 50xc2x0 C., 75xc2x0 C. and 100xc2x0 C. versus average pore radius. The n-butane working capacities are measured under the following conditions, a canister capacity is 847 ml (milliliters), adsorption are carried out up to 0.3 vol % (volume percentages) breakthrough under 100% (percentages) n-butane gas atmosphere, desorption are carried out under purge air amount of 200 Bed volume and flow rate of 101/min (liter/minutes), and the plotted data are average values of adsorption amount and desorption amount measured in fifth and sixth cycles out of six cycles of adsorption and desorption. The graph shows that the activated carbon particles having average pore radius of 10.5 Angstroms to 12.0 Angstroms provide greater working capacities than that of the activated carbon particles having average pore radius of 12.0 Angstroms or more. In FIG. 7, the activated carbon in a range RWH performs effectively when it is used with heating means. A range RNH indicates the activated carbon in case of no heater.
Although the desorption performance at normal temperatures is not high enough since the activated carbon particles have relatively small average pore radius which is in a range of 10.5 Angstroms to 12.0 Angstroms, it is possible to enhance the desorption performance by utilizing heating means for heating the activated carbon particles when desorption.
The activated carbon particles may have particle size in a range of 1.0 mm to 1.6 mm. The particle size may be defined by diameter of particles. It is possible to provide high adsorption performance. The activated carbon particles having smaller particle diameter than 1.0 mm may cause excessive pressure loss. It is possible to provide good heat conduction since it is possible to reduce gaps between the activated carbon particles. Therefore, it is possible to prevent lowering of the adsorption performance due to residual heat, because temperature of the activated carbon particles can be rapidly decreased after desorption with heating.
FIG. 8 shows refueling working capacities of activated carbon pellets and crushed activated carbon particles versus particle size. The refueling working capacities are measured under the following conditions, a canister capacity is 2000 ml (milliliters), adsorption is carried out at 25xc2x0 C. (Celsius degrees) constant up to 0.3 vol % (volume percentages) breakthrough when refueling a fuel tank of 80 liters, desorption is carried out at 25xc2x0 C. (Celsius degrees) under purge air amount of 450 Bed volume and flow rate of 201/min (liters/minute), and the plotted data are average values of adsorption amount and desorption amount measured in fifth cycle and sixth cycle out of six cycles of adsorption and desorption. The graph shows that the activated carbon having particle diameter of 1.6 mm or less provide greater working capacities than that of the activated carbon having particle diameter larger than 1.6 mm. In FIG. 8, the activated carbon in a range RIN performs effectively when it is used with heating means. A range RPA indicates the activated carbon in case of no heater.
The canister may be used for an evaporative emission control system for adsorbing and desorbing fuel vapor such as gasoline vapor. For instance, the canister adsorbs fuel vapor from a fuel tank. The heating means is deactivated and a purge valve disposed on a purge line is closed during adsorption. The canister desorbs fuel vapor by heating and purging. The adsorbed fuel vapor is purged into an intake passage of an engine when the engine is running and a negative pressure is available in the intake passage. The purge valve is opened to permit purging airflow from the canister to the intake passage. The heating means is activated to enhance desorption of the fuel vapor.